This invention relates generally to magnetic resonance imaging (MRI), and more particularly the invention relates to flow imaging using phase contrast MRI.
Magnetic resonance imaging (MRI) requires placing an object to be imaged in a static magnetic field, exciting nuclear spins in the object within the magnetic field, and then detecting signals emitted by the excited spins as they precess within the magnetic field. Through the use of magnetic gradient and phase encoding of the excited magnetization, detected signals can be spatially localized in three dimensions.
Phase contrast MRI (PC-MRI) is widely used to assess blood flow and tissue motion. The technique relies on the measurement of changes in the signal phase due to flow or motion in the presence of known linear magnetic gradient fields.
Although it is well known that non-uniformity in magnetic field gradients can cause significant image warping and require correction, little has been reported about the impact of spatial gradient field distortions on velocity encoding.
In PC-MRI, these imperfections introduce errors in velocity measurements by affecting the first moments used to encode flow or motion. Typically, based on the deviations of the actual gradient from the desired, uniform gradient, the spatial image distortions in magnitude and phase images are retrospectively corrected by image unwarping algorithms, e.g., with algorithms included in the image reconstruction software. The velocity-encoded information is moved to its correct location but the error in velocity encoding due to the local field deviation still persists. Previous work (e.g., U.S. Pat. No. 6,163,152) recognized the impact of errors in the strength of the encoding gradient and disclosed a correction for this effect. In some recent high performance gradient systems, the coil size was reduced to limit dB/dt and amplifier power. As a result, gradient uniformity has become even more degraded.